The present invention relates to a combined method of separating oxygen from an oxygen containing gas and generating power. More particularly, the present invention relates to such a combined method in which the oxygen is separated by an oxygen transport membrane purged with superheated steam and the power is generated through a Rankine cycle. Even more particularly, the present invention relates to such a combined method in which heat is transferred from an oxygen product stream produced by the oxygen transport membrane to a process fluid used within the Rankine cycle.
Integration of power and oxygen generation cycles involving the use of oxygen transport membranes are particularly attractive from the standpoint of thermal efficiency. This is because oxygen transport membranes are effective to separate oxygen in a temperature range that encompasses the high temperatures involved in many power generation cycles.
Oxygen transport membranes are formed from a variety of well known ceramics, for example, perovskites and perovskite-like materials. At a high temperature, between about 400xc2x0 C. and about 1000xc2x0 C., such ceramics are capable of conducting oxygen ions while remaining impervious to oxygen molecules and substances containing oxygen in a combined form. In an oxygen transport membrane, the oxygen in an oxygen containing gas ionizes on a cathode side of the membrane. The oxygen ions can be transported across the membrane under the impetus of a positive ratio of partial oxygen pressures applied across the membrane. The oxygen ions emerging at the opposite, anode side of the membrane, recombine to liberate electrons that are used to ionize the oxygen at the cathode of the membrane. In some materials, known as mixed conductors, the electrons are transported back to the cathode directly within the ceramic. In dual phase conductors, electrons are conducted by a metallic phase or electron conducting ceramic phase located within the ceramic.
Application of a countercurrent (to the direction of retentate side flow) steam purge to the anode of an oxygen transport membrane lowers the oxygen partial pressure along the length of the membrane to increase the driving force for oxygen transport. This permits higher oxygen recovery and/or a more efficient cycle in that the degree of compression of cathode side gases or anode side gases that otherwise would be necessary to produce the driving force is reduced and can permit withdrawing an oxygen product at elevated pressure. The molar ratio of steam to oxygen at the anode side exit of the oxygen transport membrane unit determines the maximum pressure at which the oxygen product can be recovered; the higher the ratio the higher the possible oxygen product pressure. Unfortunately significant reductions in the partial pressure of oxygen at the anode and or high oxygen product pressures require high ratios of steam to oxygen. For instance, at a retentate or cathode side air pressure of 12 Bar the partial oxygen pressure at the cathode or retentate side inlet will be about 2.4 Bar. At a minimum partial oxygen pressure ratio (driving force for ion transport across the membrane) of 1.5 at the pinch point, the corresponding required partial oxygen pressure at the permeate side will be about 1.6 Bar. If, for instance it is desired to recover oxygen at a pressure of 6 bar, the steam to oxygen molar ratio has to be about (6xe2x88x921.6)/1.6=2.75.
To recover oxygen at pressure, the permeate product stream, that consists of steam and oxygen, is cooled to condense the steam against a heat sink such as cooling water. Unfortunately, the amount of heat required to generate the large quantities of steam makes the process economically unattractive because the latent heat of condensation cannot be recovered effectively. If the steam-oxygen mixture is expanded in a turbine, the oxygen is recovered at low pressure. This is a problem when the oxygen product is subsequently required at high pressure and requires recompression. Also if one wants to avoid compressing oxygen from a high vacuum level, a significant fraction of the power producing potential, that can be recovered in the turbine, is lost.
For instance, in U.S. Pat. No. 5,562,754, air is compressed and heated in an in-line combustor. The oxygen is separated from the air in an oxygen transport membrane to produce a retentate. A stream of the retentate is expanded in a gas expander that is used to drive the air compressor and optionally, an electric generator. A steam purge is used at the anode side to produce an oxygen product containing steam that is used to preheat the feed water. Aside from such preheating, the latent heat of condensation is not recovered in the illustrated cycle and is thereby lost to the cycle. In U.S. Pat. No. 5,964,922, water is pressurized by pumping and then used as a steam purge for an oxygen transport membrane. The pressurized oxygen product that contains both permeated oxygen and steam is cooled in a water cooled or air cooled condenser to allow water to be condensed from the steam and recycled. As a result, the latent heat of condensation is thereby lost to the cooling mediums. U.S. Pat. No. 5,954,859 discloses purging the permeate side of an oxygen transport membrane with a high pressure purge gas stream containing steam to produce a high pressure gas stream containing oxygen and steam. The resultant stream is introduced into a turbine to recover shaft work. Hence if thereafter, the stream or portions of it were required at high pressure, it would require recompression with a concomitant energy outlay.
As will be discussed, the present invention encompasses an energy efficient method of producing an oxygen product stream at pressure that allows for the recovery of work from the latent heat of condensation of steam contained in such product stream. Other advantageous aspects of the present invention will become apparent from the following discussion.
The present invention provides a combined method of separating oxygen from an oxygen containing gas and generating power. In accordance with a method of the present invention, oxygen is separated from the oxygen containing gas into permeated oxygen and an oxygen depleted retentate by an oxygen transport membrane unit. The oxygen transport membrane unit includes at least one oxygen transport membrane operating at an elevated operational temperature and having a cathode side and an anode side. The anode side of the at least one oxygen transport membrane is purged with a pressurized purge stream comprising pressurized, superheated steam. A pressurized oxygen product stream is discharged from the anode side of the at least one oxygen transport membrane. The pressurized oxygen product stream comprises the permeated oxygen and the steam. At least part of the steam in the pressurized oxygen product stream is condensed by transferring heat to a process fluid that boils at a boiling temperature lower than the condensing temperature of the steam contained in the oxygen product stream. As a result, the process fluid boils and the at least part of the steam within the pressurized oxygen product stream condenses. The condensed water is separated from the pressurized oxygen product stream and energy is extracted from the process fluid as shaft work.
Preferably, the oxygen containing gas is heated prior to its being subjected to oxygen separation within the oxygen transport membrane unit. A retentate stream composed of the oxygen depleted retentate can be heated in an inline combustor by combustion of fuel supported by at least a portion of residual oxygen contained in the retentate stream to produce a heated retentate stream. The oxygen containing stream is at least partially heated by indirect heat transfer from the heated retentate stream.
The oxygen containing gas can be air that can be compressed to form a compressed air stream. A retentate stream composed of the oxygen depleted retentate can be expanded in a gas expander. Also the retentate stream can be cooled and then expanded in the gas expander to reduce its capital cost by allowing use of lower cost materials. Preferably the gas expander can then drive an air compressor to compress the oxygen containing gas. Alternately to increase power output the retentate stream can be further heated by combustion in an in-line combustor and then expanded in the gas expander.
A compressed air stream can be divided into first and second subsidiary air streams. The first subsidiary air stream can be heated through indirect heat exchange with the pressurized oxygen product stream and the second subsidiary air stream is heated separately from the first subsidiary air stream. The first and second subsidiary streams are combined prior to their being introduced into the oxygen transport membrane unit.
The separation of oxygen from the oxygen containing stream produces a retentate at the cathode side of the at least one oxygen transport membrane. A retentate stream composed of at least a portion of the oxygen depleted retentate can be introduced into an oxygen transport membrane combustor and deoxo unit so that further oxygen is separated from the retentate stream to form further permeated oxygen.
The pressurized purge stream is formed by pumping a combined water stream formed of recycled and, if necessary make-up water pumping only has to overcome pressure drop in the purge circuit. The combined water stream is vaporized and at least partially superheated to form an at least partially superheated purge stream. The combined water stream is vaporized and at least partially superheated by combusting a first fuel stream and transferring the heat of combustion to the combined water stream. A second fuel is introduced into the at least partially superheated purge stream, thereby to form the pressurized purge stream. At least a portion of the pressurized purge stream is further heated and then introduced into the oxygen transport membrane combustor and deoxo unit so that fuel reacts with the further permeated oxygen to produce heat and products of combustion. The heat can be used to at least partially heat the products of combustion and, by indirect heat transfer, at least a portion of the compressed air feed. Preferably the products of combustion are added to the pressurized purge stream to increase the volume of gases for purging the anode of said oxygen transport membrane. This reduces further the partial oxygen pressure at the anode of the oxygen transport membrane unit and also allows complete oxidation of residual fuel and partial oxidation products carried over from the oxygen transport membrane combustor.
The oxygen transport membrane combustor and deoxo unit produces a further oxygen depleted retentate. A further retentate stream composed of the further oxygen depleted retentate can be divided into first and second subsidiary retentate streams. The first subsidiary retentate stream can be expanded in a gas expander to generate power to drive said air compressor. The second subsidiary retentate stream can be recovered as a pressurized nitrogen product. The first subsidiary retentate stream can be further heated by combustion within an in-line combustor prior to expansion. Part of the second subsidiary air stream can be combined with the first subsidiary retentate stream upstream of the in-line combustor to support combustion of a fuel and thereby to add further heat and mass to the first subsidiary retentate stream prior to its being expanded.
In any embodiment of the present invention, the process fluid can be water and the steam in the pressurized purge stream can be at a higher pressure than the process fluid. The power can then be extracted from the process fluid in a Rankine cycle in which the process fluid is pumped in the form of the liquid condensate to a pressure lower than that of the purge stream to create a pressurized liquid. The pressurized liquid is vaporized by indirect heat exchange with at least part of the condensing steam in the purge stream in a reboiler-condenser. The process fluid can be superheated and the process stream can be expanded in a steam turbine after having been superheated. The power can be extracted from shaft work created in the turbine. Thereafter the process fluid exhausting from the steam turbine can be condensed to produce the liquid condensate.
The pressurized purge stream can at least be made up of recycled water and, if necessary make-up water to form the water stream. It should be noted that, if products of combustion are added to the pressurized purge stream, the water from the products of combustion may produce a surplus of water requiring removal of this excess.
The water stream can be vaporized and superheated to form the pressurized purge stream. At least a portion of the heat required for the vaporization and superheating of the water stream can be provided by combustion in a boiler-superheater. The process fluid is superheated by the combustion within the boiler-superheater.
The water for the pressurized purge stream can be pumped, vaporized, and superheated to form the pressured purge stream. The pressurized purge stream thus formed can be expanded in a turbine to produce additional power. After expansion, the pressurized purge stream can be reheated and used to purge the anode side of the at least one oxygen transport membrane. The pressurized purge stream is indirectly heated along with said first subsidiary air stream by the oxygen product stream. Fuel can be added to the pressurized purge stream after having been expanded. Thereafter, the pressurized purge stream is introduced into the oxygen transport membrane combustor deoxo unit as a reactive purge to produce heat and products of combustion which together with steam contained in said pressurized purge stream are used to purge the anode side of the at least one oxygen transport membrane.
The steam turbine of the Rankine cycle can operate at an exhaust pressure of significantly less than about 14.7 psia since the condensing temperature is only limited by the temperature of cooling media in the condenser. At least a portion of the heat for vaporizing and superheating said purge steam can be provided by recovery of heat from the exhaust of the expander.
In the present invention since the pressurized oxygen product stream is not simply expanded to extract power, but rather, is used to supply heat to a Rankine cycle, a pressurized oxygen product that is saturated with water can be produced or a pressurized oxygen-steam mixture produced at a desired steam/oxygen molar ratio. If required the moisture saturated oxygen stream can be dried by conventional means. Steam oxygen mixtures can be used in downstream processes such as coal gasification or autothermal reformers. Withdrawing product at elevated pressure reduces or eliminates capital and energy intensive oxygen compression. At the same time, energy can be efficiently extracted by the Rankine steam cycle.